Electronic geolocation apparatus allows local position on earth to be determined with considerable accuracy (typically +/-100 meters) using signals received, for example, from Global Positioning System (GPS) satellites orbiting the earth. Hand-held GPS receivers are available from a number of commercial sources and are much used by hunters, hikers, mariners, surveyors, military personnel and other persons who have need for instantaneous geolocation data. Multi-channel receivers, i.e., receivers able to simultaneously track several satellites at the same time, are common. Geolocation apparatus of this type is portable, self-contained, comparatively inexpensive (e.g., currently about $10.sup.3), and in many cases hand holdable. For example, the TRAXAR.TM. and Commando SPS.TM. six channel GPS navigator receivers from Motorola, Inc., Scottsdale, Ariz. weight about 0.5 Kgm and have a volume of about 4.times.10.sup.2 cc and simultaneously track up to six satellites.
Many GPS units also provide "course-to-steer" information, that is, the heading to be followed to go from one location to another. For example, the user can input latitude and longitude of a desired waypoint, whereupon, the unit will calculate the course-to-steer to go from the present position (determined by the GPS receiver) to the indicated waypoint (supplied by the user). The term "waypoint" generally refers to a desired destination or navigational reference point to be reached, which may be a final destination or one of a series of intermediate destinations or navigational reference points to be traversed. More sophisticated units are able to display the course-to-steer referenced to true north or magnetic north, as the user desires.
While existing GPS units can tell the user a desired course-to-steer to reach a particular destination, e.g., "steer 090 degrees", and they can tell the user what course has been steered, e.g., by keeping track of and displaying successive locations, they are unable to tell the user what course is currently being steered. Thus, in order to take advantage of course-to-steer information the user must still have some independent means (e.g., a compass) for determining current direction or heading. Without landmarks or a compass or equivalent, the course-to-steer information provided by a GPS receiver is useless.
For example, consider a mariner at sea in a dense fog. His geolocation receiver gives the precise location of the boat which can be marked on the mariner's chart. From the same chart the location of a safe harbor and the course-to-steer to reach the harbor can also be determined. But, if the compass is broken and the stars or sun or moon cannot be seen, the mariner cannot determine his current heading. There is no local frame of reference by which the mariner can tell whether the boat is traveling along the desired course-to-steer or along some other direction. The mariner knows where he has been, where he is and, presumably, where he wants to go, but not where he is actually going. Thus, the typical geolocation receiver by itself does not provide all of the information that the mariner (or other traveler) needs for navigation and some other means of determining heading is required.
The lack of navigational orientation information from GPS receivers has long been recognized and several solutions have been described to overcome this, as for example, in U.S. Pat. Nos. 4,599,620 to Evans and 4,881,080 to Jablonski, and in an article by DiNardo et al., entitled "GPS Measurement of Attitude", NASA Tech Briefs, August 1992, page 34.
While these prior art approaches to providing orientation information, e.g., vessel heading or direction, based on GPS are successful for ships, planes and other large objects, they are not suitable for use with portable apparatus, especially hand-held apparatus. With hand-held portable apparatus, it is not practical to have, for example, a two meter rotating antenna or multiple antennas separated by a baseline measuring 32 to 100 meters in length. Thus, a need continues to exist for a means and method for obtaining orientation information (i.e., current heading) from portable geolocation apparatus, especially GPS geolocation apparatus. It is especially desirable to be able to provide such information with a single hand-held portable apparatus.
As used herein, the words "heading" and "course" are used interchangeably to indicate vectors pointing in a particular spatial direction. The words "level plane" or "horizontal" are intended to refer to a plane approximately perpendicular to the earth's gravity vector.